Biodigestion of kitchen waste
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Biodigestion of kitchen waste |
Final report prepared by The University of Southampton and Greenfinch Ltd
Funded by BIFFAWARD and South Shropshire District Council through the Landfill Tax Credit Scheme
CONTENTS
Summary………………………………………………………………………………………...4
1. Introduction…………………………………………………………………………………...5
2. Aims and objectives…………………………………………………………………………..6
3. Methodology……………………………………………………………………………….....6
4. Results and discussion………………………………………………………………………..8
4.1 Kitchen waste collection…………………………………………………………….8
4.2 Pre-processing of kitchen waste………………………………………………........11
4.3 Optimisation of mesophilic and thermophilic digestion…………………………...12
4.4 Pathogen control in mesophilic and thermophilic digestion……………………….17
Conclusions…………………………………………………………………………………….24
FIGURES
Figure 1: The thermophilic and mesophilic digester systems,
showing the mixed feed tanks, digester and collection tanks…………………………………...7
Figure 2: Schematic diagram of the anaerobic digester system………………………………...8
Figure 3: Weight of kitchen waste collected from Burford village and Tenbury Wells………..9
Figure 4: Average composition of source separated waste (by weight)……………………….10
Figure 5: Frequency of particle size (mm) of 100 samples of shredded raw
kitchen waste, mixed feed and digestate……………………………………………………….11
Figure 6: pH and alkalinity of the mesophilic and thermophilic digesters…………………….13
Figure 7: Concentration of volatile fatty acids, biogas yield, percentage of methane and
retention time in the (a) mesophilic and (b) thermophilic digesters…………………………..15
Figure 8: Concentration of Salmonella in the (a) mesophilic and (b) thermophilic anaerobic
digester systems………………………………………………………………………………..19
Figure 9: Concentration of E. coli in the (a) mesophilic and (b) thermophilic anaerobic digester
systems. …………………………………………………………………………................….20
Figure 10: Concentration of Entercocci in the (a) mesophilic and (b) thermophilic anaerobic
digester systems. ……………………………………………………………………………....21
TABLES
Table 1: The effect of hygiene measures on concentrations of Salmonella,
E. coli and Enterococci in mesophilic and thermophilic digestate…………………………….23
References…………………………………………………………………………………...…25
APPENDICIES
Appendix 1: Analytical methods………………………………………………………………26
Appendix 2: Lithium Tracer Test……………………………………………………………...32
Appendix 3: Kitchen Waste Collection……………………………………………………......36
Appendix 4: Hygiene Regime……………………………………………………………........41
SUMMARY
Aim of the project
The aim of the project was to establish whether anaerobic digestion, possibly combined with heat treatment, is a safe means of processing kitchen waste and restaurant waste, to determine whether the disposal of the stabilised and sanitised digestate would meet the draft EU Animal By-Products Regulation.
Methodology
Kitchen waste was collected on a weekly basis from the village of Burford, Shropshire. The waste was shredded at the anaerobic digestion site and treated in two digesters, one operating at mesophilic temperature (36.5oC) and the other at thermophilic temperature (56oC). The digesters were operated on a six hour cycle and the volatile solids loading was varied to optimise the system. Samples were taken from the digesters, feedstock and collection tanks to assess (among others) biogas yield, volatile solids reduction, and presence of bacteria and pathogens.
Main findings
Source separation of the organic fraction of municipal waste yields on average 2.9 kg per household per week with less than 2.5% contamination. In all, 23.2 tonnes of kitchen waste was diverted from landfill. The average moisture content of the source-separated kitchen waste was 78.4%, and volatile solids were found to be 92.4%. Thermophilic digestion of kitchen was found to be less stable than mesophilic digestion. Mesophilic digestion medium has a greater buffering capacity and is more robust to changes or accumulation of inhibitory chemicals. The biogas in both systems comprised of 58% methane. The mesophilic system produced 164 m3
per tonne, whereas the thermophilic system produced 157m3 per tonne. Kitchen waste was collected on a weekly basis and contained pathogens such as Salmonella, Escherichia coli and Enterococci. Thus, this waste would require further treatment before it could be applied to agricultural land to meet the draft EU Regulations. Mesophilic digestion produced a 3.0 log reduction for E. coli, 2.8 log reduction of Enterococci and 1.4 log reduction of Salmonella. Overall, thermophilic digestion produced a 5.07 log reduction for E. coli, 4.0 log reduction of Enterococci and 2.2 log reduction of Salmonella. Digestate from the collection tanks in both the mesophilic and thermophilic systems still contained Salmonella, and would therefore require further treatment prior to spreading on agricultural land. The introduction of a hygiene regime reduced the presence of Salmonella, E. coli and Enterococci in thermophilic digestate. Laboratory-scale pasteurisation at 70°C for at least 1 hour effectively eliminated Salmonella, E. coli and Enterococci. When the digestate is stored at 20°C, Salmonella persists for 10 days and E.coli for 21 days, but Enterococci was still present after 3 months.
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